1. Field of the Invention
The present invention relates to an optical film, diffusion sheet, reflector, surface light source device and liquid crystal display. More particularly, it relates to an optical film or the like having an optical pattern formed thereon. Further, it relates to a surface light source device or liquid crystal display utilizing the optical film or the like.
2. Description of the Prior Art
FIG. 1 is an exploded and partially cutaway perspective view showing a configuration of a conventional surface light source device 1 used for a liquid crystal display or the like. In this surface light source device 1, a diffusion sheet 3 is laminated above a light guide plate 2 (an observer side) made of a transparent material having a great refractive index, on which two prism sheets 4 and 5 are laminated. Arranged below the bottom surface of the light guide plate 2 is a reflection sheet 6 made of a white sheet or the like. Further, a light source 7 composed of a cold-cathode tube is arranged at the side of the light guide plate 2.
A dot pattern 8 of diffused reflective ink as shown in FIG. 2 is printed on the bottom surface of the light guide plate 2 or a diffusion pattern by an optical pattern is recessedly provided thereon. Further, each of the above-mentioned prism sheets 4 and 5 has arranged on its top surface a prism having a triangular-shaped section that is uniformly extended. The prism sheet 5 is positioned on the prism sheet 4 so as to be rotated at an angle of 90° with respect to the prism sheet 5.
In this surface light source device 1, light emitted from the light source 7 enters into the light guide plate 2 from the side surface of the light guide plate 2, and then spreads all over the light guide plate 2 while repeating a total reflection at the top surface and the bottom surface of the light guide plate 2. As shown in FIG. 3, when light L that is transmitted as repeating the total reflection in the light guide plate 2 comes in contact with the dot pattern 8 to thereby be diffusedly reflected, an incident angle to the top surface of the light guide plate 2 is changed. When the light L incident to the top surface of the light guide plate 2 is introduced with an incident angle smaller than a critical angle of the total reflection, it is emitted to the outside from the top surface (light-emitting surface) of the light guide plate 2. Further, since the amount of the reaching light L becomes small as it becomes far away from the light source 7, the density of the dot pattern 8 is increased as it becomes far away from the light source 7 in order that the light is emitted from the top surface of the light guide plate 2 with uniform brightness.
FIG. 4 represents a movement of the light L by the light guide plate 2 having a diffusion pattern 9 recessedly provided at its bottom surface. In such a light guide plate 2 too, the transmitted light L while repeating the total reflection in the light guide plate 2 changes its incident angle to the top surface of the light guide plate 2 when it comes into contact with the diffusion pattern 9 to thereby be reflected as shown in FIG. 4. The light L incident on the top surface of the light guide plate 2 with an incident angle smaller than the critical angle of the total reflection is emitted to the outside from the top surface (light-emitting surface) of the light guide plate 2.
FIG. 5 is a view for explaining a directional characteristic of the light in this surface light source device 1. Since the light is emitted from the top surface of the light guide plate 2 with the movement shown in FIGS. 3 and 4 in the surface light source device 1, the light emitted from the top surface of the light guide plate 2 becomes light approximately parallel to the top surface of the light guide plate 2 and having a strong directivity as represented by a directional characteristic Pa. The light having a strong directional characteristic is diffused by passing through the diffusion sheet 3, thereby becoming diffused light having a directional characteristic Pb like Lambert light. Accordingly, the use of the diffusion sheet 3 improves non-uniform brightness of the light guide plate 2. Moreover, the light emitted from the top surface of the light guide plate 2 is diffused by the diffusion sheet 3 for widening the direction of the light, with the result that it is difficult to be totally reflected at the bottom surface of the prism sheet 4, thereby increasing the light amount introduced into the prism sheet 4.
However, even if the directivity is decreased by the passage through the diffusion sheet 3, the center of the directional characteristic of this light is inclined from the direction perpendicular to the diffusion sheet 3. When the diffused light passing through the diffusion sheet 3 passes through two prism sheets 4 and 5 each having different direction, the light passing through the prism sheets 4 and 5 is aligned again so as to become light with strong directional characteristics Pc and Pd having a strong directivity in two directions (direction wherein the prism of each prism sheet 4 and 5 is arranged), as well as the light-emitting direction is aligned in the direction perpendicular to the prism sheets 4 and 5.
FIG. 6 is a view for explaining a function of each of the prism sheets 4 and 5. FIG. 6A represents the movement of the light L when the light L passing through the diffusion sheet 3 is diagonally introduced into the bottom surface of one prism sheet 4. The light L introduced into the prism sheet 4 from the bottom surface of the prism sheet 4 is refracted at the bottom surface of the prism sheet 4 and is directed upward, and further refracted upon being emitted from the slope of the prism, thereby being emitted toward the direction perpendicular to the prism sheet 4.
On the other hand, the light L vertically introduced toward the bottom surface of the prism sheet 4 from the diffusion sheet 3 is totally reflected (returningly reflected) twice at the slope of the prism and then is returned toward the diffusion sheet 3 and the light guide plate 2 as shown in FIG. 6B. The light L returned to the diffusion sheet 3 and the light guide plate 2 is reflected by the reflection sheet 6 to thereby be emitted again from the top surface of the light guide plate 2.
Therefore, a part of the light L diffused by the diffusion sheet 3 and incident on the prism sheets 4 and 5 is emitted in the direction perpendicular to the prism sheets 4 and 5, while another part of the light L is returningly reflected by the prism sheets 4 and 5 to thereby be returned to the light guide plate 2, reflected by the reflection sheet 6, and then, passes through the diffusion sheet 3 to be introduced again to the prism sheets 4 and 5. As a result, the light L passing through the diffusion sheet 3 is aligned in the direction perpendicular to the prism sheets 4 and 5 to be emitted with high efficiency.
FIGS. 7A, 7B and 7C are views each showing an angle distribution of the light L emitted from the light guide plate 2, an angle distribution of the light L passing through the diffusion sheet 3 and an angle distribution of the light L passing through the prism sheet 4. Each view represents the brightness of the light L emitted in the direction at an angle of θa, θb or θc with respect to a vertical axis N as shown in FIG. 8. According to FIG. 7A, the brightness of the light L shows a great peak in the vicinity of 60°, that is sufficiently agreed with the state in which the light L emitted from the top surface (light-emitting surface) of the light guide plate 2 is normally emitted in the direction inclined from the vertical axis N (generally about 60°±15°).
Moreover, the peak of the brightness is moved to the position of approximately 40° in FIG. 7B. As the height of the brightness peak is lowered, the brightness becomes gentle in total to thereby be almost uniform. This corresponds to the fact that, when the light L passes through the diffusion sheet 3, non-uniform brightness is improved and the direction of the optical axis also approaches to the direction of the vertical axis N.
Further, the peak of the brightness is moved to the position of approximately 30° wherein the brightness in the direction at an angle of 0° (the direction of the vertical axis N) becomes great. Accordingly, it is understood from FIG. 7C that the use of the prism sheets 4 and 5 can align the emitting direction of the light L to the direction perpendicular to the prism sheets 4 and 5.
As described above, the diffusion sheet or the prism sheet arranged at the light-emitting side of the light guide plate plays an important role for reducing non-uniform brightness of the emitting light or for increasing a frontal brightness in the surface light source device.
However, the spread of the light diffused by the diffusion sheet 3 is great in the above-mentioned surface light source device, so that the light L diffused in the direction shown in FIG. 9 is transmitted through the prism sheets 4 and 5 to thereby be emitted in the diagonal direction. Such light L is a loss, thereby entailing a problem of decreasing the frontal brightness of the surface light source device.
Further, the diffusion sheet or prism sheet separately formed is required to be laminatedly arranged on the light guide plate in the surface light source device having the above-mentioned configuration shown in FIG. 1, so that there is a limitation on decreasing a thickness of the surface light source device. Moreover, the number of the components is increased in the surface light source device, thereby entailing a problem of increasing the number of assembling or increasing cost.